Data Category 15 is used in all programs in the Aqwa suite, except Aqwa-Line, to input the 'starting conditions' for the analysis. Although data categories for each type of analysis are similar, they are documented separately as each requires information appropriate to the type of analysis. This description of Data Category 15 is appropriate to Aqwa-Drift only.

Aqwa-Drift solves the second order differential equations of motion by integrating them to form a time-history of motions. The program requires the initial conditions in order to begin the integration. These initial conditions are referred to as starting conditions.

As the differential equation is second order, the solution requires two initial conditions which may be input by the user. These are the position and velocity at the start of the time-history in all 6 degrees of freedom. However, default starting conditions will be assumed by Aqwa-Drift if the user omits any, or all (the latter by entering NONE for the Data Category Keyword) of the data records in this data category.

'Fast', 'Slow' and 'Total' Positions and Velocities

The total motion of a structure in Aqwa-Drift is the sum of its 'slow' drift motion and the 'fast' wave frequency motion. The program, in general, needs the initial conditions of the structure for both the fast and slow motions. Also it is important that the structure experiences as small a transient as possible. This can only be achieved if the program has appropriate values of slow and fast initial conditions. This is not as complicated as it sounds since the program automatically performs the difficult calculations.

The initial conditions for the slow motion are relatively intuitive since these relate to the general motion of the structure about its equilibrium position as predicted by Aqwa-Librium. The fast motions, however, are in response to the wave frequency forces which are randomly phased, so that the user is generally unable to specify them. In this case, the program automatically computes the correct fast motion from the information it has concerning the random waves and the response amplitude operators of the structure.